Antioxidant supplements that are supposed to boost health and slow ageing could in fact spur the spread of melanoma, the deadliest type of skin cancer, researchers said Wednesday.

The findings support recent studies showing that an over-the-counter vitamin and another drug containing antioxidants can cause a jump in the number of tumours and hike their aggressiveness.

In the new study, published in Nature, scientists in the United States demonstrated that human melanoma cells spread in some experiments about two months earlier in mice injected with antioxidant N-acetylcysteine (NAC) than in those that were not.

Antioxidants allow the body to prevent DNA damage from chemicals known as free radicals, and are produced naturally by humans and found in leafy greens, vegetables or fruit.

Metastasis is the spread of cancer from one part of the body to another, and leads to death in most people fighting the disease.

It appears antioxidants help cancer cells by fighting against a type of molecule in the human body that can attack or damage them as they metastasise, the scientists said.

The results have not yet been tested on people, but researchers suggest that cancer patients should consider not supplementing their diets with the oxidation-fighting substances.

The notion that antioxidants are good for you has gained such force that they have been given to cancer patients in clinical trials, said study co-author Sean Morrison, who heads a research institute at the University of Texas Southwestern in Dallas.

"Some of those trials had to be stopped because the patients getting the antioxidants were dying faster," he said.

In a previous study on mice, Swedish researchers said antioxidants, including vitamin E, caused a three-fold increase in the number of cancer tumours and led the rodents to die twice as fast.